Somatic stem cells are required throughout life to maintain many regenerative mammalian tissues. Stem cells undergo self-renewing divisions and differentiate to form mature progeny that replace cells lost to turnover, injury, or disease. In the blood, hematopoietic stem cells (HSCs) remain quiescent in a specialized niche in the bone marrow and divide infrequently. In response to hematologic injury, such as bleeding, chemotherapy, infection, or pregnancy, HSCs activate and mobilize to sites of extramedullary hematopoiesis. Active HSCs proliferate to expand the pools of HSCs and restricted progenitors to restore homeostasis and normal blood cell counts. Despite the difference in their proliferative state, both quiescent and activated HSCs retain the capacity to reconstitute hematopoiesis upon transplantation into irradiated mice. While many of the mechanisms that regulate the maintenance of quiescent stem cells have been identified, little is known about the mechanisms that regulate HSC activation and how mechanisms that maintain active HSCs are distinguished from those that maintain quiescent HSCs. Both quiescent and dividing HSCs exhibit a lower rate of protein synthesis as compared to other hematopoietic cells, and genetic changes that increase their rate of protein synthesis impair HSC function. Yet, the rate of protein synthesis modestly increases in dividing as compared to quiescent HSCs. This raises the question of how this increased rate of protein synthesis during cell division is reconciled with the need to sustain low levels of protein synthesis for HSC maintenance. What changes during HSC activation? Heat shock factor 1 (HSF1) directs the cellular response to a variety of stresses including proteotoxic stress, by transcriptionally upregulating the expression of protein chaperones. HSF1 regulates a broad range of biological and cellular processes that could potentially promote the maintenance of HSC function after activation including metabolism, heat shock response, and proteostasis. Nonetheless, whether HSF1 is required for the maintenance of HSCs, HSC activation in response to stress, or for the regulation of proteostasis in activated HSCs is unknown. In this proposal, I will test whether HSF1 regulates HSC function under steady state conditions and in response to hematopoietic stresses by conditionally deleting HSF1 from the hematopoietic system. I will test whether HSF1 protects HSCs from proteotoxic stress or regulates protein synthesis in HSCs. I will also identify the genes whose expression is regulated by HSF1 in quiescent and active HSCs using RNA-seq and ChIP-seq methodology to identify the mechanisms by which HSF1 acts. The results of these studies will assess how HSCs regulate protein synthesis and how they increase their rate of protein synthesis upon activation.